J. Cent. South Univ. Technol. (2010) 17: 400-405

DOI: 10.1007/s11771-010-0059-6                                                                                                                   

Double pull specimen more suitable for measuring bond-slip relationship of FRP-to-concrete interface

YANG Qi-fei(杨奇飞), LU Xiao-hua(陆小华), XIONG Guang-jing(熊光晶)

Department of Civil Engineering, Shantou University, Shantou 515063, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2010

Abstract: A new “conceptual” design named “double pull” specimen was proposed in order to measure the bond-slip (δ-τ) relationship of fiber reinforced polymer (FRP)-to-concrete interface more accurately. A finite element analysis (FEA) was performed for preliminarily evaluating the suitability of the proposed conceptual double pull specimen. Through the FEA, it was indicated that the FRP-to-concrete interface of the proposed conceptual specimen might subject to a much higher load level than that of the most commonly used simple shear specimen, showing a great potential for measuring δ-τ relationship more accurately. In the light of the conceptual specimen, a kind of “practical” double pull specimen was developed and proved to be more suitable for measuring δ-τ relationship through an exploratory experimental study with 20 specimens. Consequently, an experimental program with 10 double pull specimens was performed for measuring the ultimate slip δ_u which was difficult to capture by using the existing specimens. It is shown that the range of δ_u is 0.31-0.52 mm based on the test results. The suggestion for improving the measure method is also put forward.

Key words: fiber reinforced polymer (FRP); concrete; bond-slip; ultimate slip; double pull specimen; extensometer

1 Introduction

Fiber reinforced polymer (FRP) composites have been widely used for strengthening reinforced concrete (RC) elements in the past decade [1]. With the rapid development of this new technology, many issues related to the structural performances of FRP strengthened RC elements have been studied. Among them, the study on the interfacial bond between the externally bonded FRP sheets and concrete may be the most fundamental one because it plays a key role in the composite performances and the durability of RC structures after being strengthened. In order to evaluate the interfacial bond mechanisms quantitatively and carry out numerical simulation for FRP sheets strengthened RC elements, defining an accurate bond-slip (δ-τ) constitutive law has become a main task among the bond issues studied in recent years.

A fairly large number of experimental researches on FRP-to-concrete interface under shear have been carried out in the past decade, and many δ-τ curves have been proposed [2-5]. It is generally agreed that a δ-τ curve is clearly nonlinear. It rises very fast initially with a large initial stiffness. After the bond stress reaches the peak value δ_u, the curve descends slowly. Such δ-τ curves can be characterized by three key parameters: the peak bond stress τ_u and the corresponding slip δ₀ (representing the end of the ascending branch of the curve), as well as the ultimate slip δ_u (expressing the end of the descending branch and indicating the initiation of debonding).

It was reported that the ascending branch, the values of δ₀ and τ_u of a δ-τ curve could be measured more accurately by using the most commonly used simple shear specimens [6-8]. The peak bond stress τ_u was in the range of 4 to 6 MPa, the corresponding δ₀ was the range of 0.02-0.05 mm [9]. The descending branch and the ultimate slip δ_u, however, have been difficult to be measured accurately by using the existing specimens, even though a number of researchers have been exerting great efforts to improve the measure methods [4, 6-8]. This is because the entire FRP-to-concrete interface of the existing specimens generally debonded suddenly at a low level of loading [2-8], leading to little chance for capturing a complete descending branch of the δ-τ curve.

In view of that the accurate δ_u values could not be obtained by using the existing specimens, a number of researchers have predicted the values of δ_u based on different estimations or theoretical considerations. CAO et al [6] estimated that δ_u was around 0.15 mm by referring the whole displacement process of the loaded end of the FRP. LU et al [7] predicted δ_u of 0.2 mm by adjusting a few key parameters through a meso-scale finite element model and experimental results. NAKABA et al [8] estimated a δ_u value of 0.5 mm by incorporating test results with analyzing results of elastic mechanics. It is obvious that these predicated values differed from one another to a large extent. Thus, further investigations need to be conducted on the accurate measurement of δ-τ relationships.

Noticing that the entire FRP-to-concrete interface of the existing specimens often debonded suddenly at a low level of loading [5-6], a new kind of specimen was attempted to be developed by the writers. The new specimen was expected to be able to subject to a much higher loading without sudden entire debonding, and consequently measure the descending branch of the δ-τ curve accurately. Three parts were included in this work. In the first part, the experimental studies using existing specimens were assessed, and a new kind of “conceptual double pull specimen” with a great potential to overcome the weakness of the existing specimens was proposed. A numerical study was followed to initially judge whether the FRP-to-concrete interface of the proposed conceptual specimen may subject to a much higher loading than that of the existing simple shear specimens. In the second part of this work, an exploratory experimental study was carried out to transform the conceptual specimen into the “practical” one with higher suitability for measuring δ-τ relationship. In view of that it was difficult to capture the ultimate slip δ_u by using the existing specimens, an experimental program with 10 double pull specimens was carried out for measuring δ_u in the last part of this work.

Other similar specimens have not been reported by other researchers. Therefore, the newly proposed double pull specimen is recommended for measuring the δ-τ relationship of FRP-to-concrete interface more accurately in future research; the measured range of the ultimate slip δ_u in this work is expected to contribute to a better understanding of the δ-τ relationship.

2 Proposed conceptual “double pull” specimen

Simple shear specimens (Fig.1) have been most commonly used to investigate the δ-τ relationships. However, the entire FRP-to-concrete interface in AB range (Fig.1) of the specimens often debonded suddenly at a low level of loading [2-8]. As a result, it was difficult to capture the complete descending branch of a δ-τ curve, and the δ_u value had to be predicted based on estimations or theoretical considerations. Three typical δ-τ curves [6-8] are chosen from the existing researches [3-11] and shown in Fig.2. It can be seen that the three predicted δ_u values had great differences.

In order to measure the δ_u values, a new kind of specimen should be developed. The inspection of the debonding features of FRP strengthening beams was taken to be the first step. Beam end debonding [12] and intermediate debonding (beam segment between two flexural cracks) [13] may occur in FRP strengthening beams. However, the loading level corresponding to intermediate debonding is much higher than that corresponding to end debonding [14]. The main reason may be that the working state of the FRP-to-concrete interface of beam intermediate (loading from both sides) is very different from that of beam end (loading from only one side).

Fig.1 Sketch map of existing commonly used simple shear specimen

Fig.2 Existing typical δ-τ curves

In the light of the working state of the beam segment between two flexural cracks, a new “conceptual” specimen shown in Fig.3 was proposed. Since the loads were applied on both sides of the proposed conceptual specimen, the specimen was named as “double pull” specimen. The conceptual specimen was expected to have a great potential for measuring δ_u accurately.

Fig.3 Sketch map of proposed conceptual double pull specimen

3 Preliminary comparison studies on different kinds of specimens

Finite element analysis (FEA) has been widely used for preliminary mechanical analysis of complex problems [15-16]. To compare the difference between the debonding mode of the existing simple shear specimen (Fig.1) and that of the proposed double pull specimen (Fig.3) preliminarily, FEA was also employed in this work.

3.1 Finite element modeling

The software ANSYS was used for the analyses. The existing simple shear specimen illustrated in Fig.1 was taken to build the finite element model, as shown in Fig.4(a). For the properties of symmetry, half of the proposed “double pull” specimen shown in Fig.3 was modeled, as shown in Fig.4(b).

Fig.4 Finite element models for different kinds of specimens:  (a) Existing simple shear specimen; (b) Proposed double pull specimen (Unit: mm)

The concrete was modeled using 4-node plane stress elements (Plan 42 element). FRP was modeled using 2-node bar elements (Link 8 element). The bond-slip behavior of FRP-to-concrete interface was modeled using 2-node spring elements (Combin 39 element).

The concrete and FRP were assumed to be linear elastic materials, and the material parameters as well as specimen dimensions adopted by the model are listed in Table 1. The constitutive law proposed by NAKABA et al [8] with τ_f=5 MPa, δ_f=0.03 mm and δ_u=0.5 mm (see Fig.2) was used for FRP-to-concrete interface.

Table 1 Material parameters for numerical study

3.2 Results of numerical study

The FEA showed that the entire FRP-to-concrete interface of the existing simple shear specimen (Fig.4(a)) debonded when P reached about 10% of P_u (the ultimate load carrying capacity of FRP).

For the double pull specimen (Fig.4(b)), however, when P arrived at 30% of P_u, point B of the FRP-to-concrete interface just reached the initial local debonding stage. With the increase of the loading, the debonding zone was gradually propagated from B towards the center of the specimen (see Fig.4(b)). When P arrived at 100% of P_u, the debonding length was extended up to 100 mm, and however, the entire FRP-to- concrete interface debonding did not appear. This is because for the symmetry, the slip δ of the central part around point C was always kept to be less than δ_u (see Fig.4(b)). It can be inferred that the double pull specimen may have a great potential for measuring δ_u accurately.

4 Development of practical double pull specimen

A pilot experiment was carried out for examining the feasibility of the initially proposed double pull specimens (Fig.3). The test result showed that the two ends of the concrete block were prone to crack along the diagonal dotted lines (see Fig.5) before local FRP-to- concrete debonding. The specimen was then improved. The concrete and FRP were separated by plastic tape on the edge of concrete block to form 100 mm-long unbonded regions as shown in Fig.5. The second pilot experiment showed that, the concrete cracking in the improved specimens was avoided. Twenty specimens were tested in the development process, and no specimen showed any sign of entire FRP-to-concrete debonding. Thus, the feasibility of the double pull specimens was proved based on both the experimental study and the numerical analysis (see section 3.2)

Fig.5 Sketch map of design of improved double pull specimen (Unit: mm)

5 Test program using practical double pull specimens

5.1 Materials

The designed concrete cube strength of the specimens was 40 MPa. The actual 28 d cube strength was 45.7 MPa. From specimen tests the tensile strength and elastic modulus of CFRP were found to be 2.647 and 211 GPa, respectively. The elastic modulus of the epoxy resin was 2.1 GPa according to the manufacturer.

5.2 Specimen production and test

Ten “double pull” specimens were prepared as indicated in Table 2. Fig.5 shows the dimensions of the specimens. Each specimen had a 400 mm-long concrete block with a 100 mm×100 mm cross section. One layer of CFRP sheet, 40 mm-wide and 0.11 mm-thick, was bonded with epoxy resin on the concrete block.

Table 2 Specimen number and measured values of δ_u

Twenty one strain gauges (with 7 mm gauge length) were applied at 10 mm spacing in the bonded region (Fig.6(a)). The outliers of the CFRP (see Fig.6(a)) were gripped by the test machine. Axial tension was applied until the P reached about 70% of ultimate load carrying capacity of FRP. The strain values were automatically recorded by a data logger at the levels of 10%, 20%, …, 70% of ultimate load.

Fig.6 Strain and slip distributions of AB region of specimen X₁ at different loading levels: (a) Arrangement of strain gauges; (b) FRP strain distribution; (c) δ distribution

6 Test results and discussion

6.1 Features of local debonding

Local interfacial debonding in the concrete layer adjacent to the adhesive layer near points A and B (Fig.7) was observed in every specimen. The broken lines could be identified to be a typical fracture plane of local debonding (Fig.7), and this plane was generally slightly wider than the width of the FRP. The fracture plane propagated from the loaded ends toward the middle of the specimens as loading increased, but no entire FRP-to-concrete interface debonding appeared in any specimen. The “double pull” specimen (Fig.5), therefore, is more suitable for measuring δ-τ relationship than the existing “simple shear” specimen (Fig.1).

6.2 Strain and slip distributions of FRP

The strain distributions of the FRP of specimen X₁ at different loading levels are plotted in Fig.6(b). Local slips were computed by integrating the strains measured on the surfaces of the FRP sheets at 10 mm interval. Slip δ_i at the ith strain gauge location is given as follows.

            (1)

                                     (2)

where ε_i is CFRP sheet strain measured by the ith strain gauge (Fig.6(b)).

The obtained δ distributions of specimen X₁ at different loading levels are shown in Fig.6(c). It can be seen that the FRP strains at gauges 10 and 12 in the middle of the specimen (Fig.6(b) were 5.013×10^-3 and 4.850×10^-3 respectively at the last loading level, indicating the FRP stresses of 1.057 74 and 1.023 35 GPa respectively. The corresponding slips of FRP-to-concrete at the points of gauges 10 and 12, however, were only 0.049 and 0.048 mm, respectively. Because they were much less than the δ_u values of 0.35 and 0.41 mm (Table 2), no debonding happened in the range of A′B′ (Fig.7). Evidence indicates that loading from the both sides of a specimen is a valid way to prevent the FRP-to-concrete interface from entire debonding, and therefore, to provide a great potential for accurately measuring δ-τ relationship.

Fig.7 Debonding feature of double pull specimen

6.3 Measured ultimate bond slip δ_u

According to the existing researches [17-18], the slips of points A′ and B′ (Fig.7) were equal to δ_u. The tested δ_u values of the 10 specimens are listed in Table 2. They ranged within 0.31-0.52 mm. It can be seen that δ_u was underestimated by most existing theoretical predictions(less than 0.2 mm) [6-7], and only the predicted value (0.5 mm) provided by NAKABA et al [8] showed an agreement with the measured δ_u of this test. A great scatter feature of the measured δ_u values could be observed. The influences of the uncertainties of various material properties (concrete, resin and FRP), bond layer thickness and bonding process were responsible for the scattering. Evidence also indicates that the double pull specimens are suitable for investigating the influence of various parameters on δ_u.

6.4 Discussion on measure method

The conventional method to obtain δ was divided into two steps. The strain distributions of FRP were measured by many strain gauges mounted on FRP sheets in the first step; and δ was predicted by a numerical integration of the measured axial strains of FRP in the second step [2-4]. This method was also applied in this work. Because δ was not directly measured, a relatively large error might not be avoided. It is necessary to find a better measure method to avoid such an error. It was reported that δ could be measured by extensometers directly [4]. A further investigation on measuring δ by combining extensometer with strain gauges should be conducted in the future.

7 Conclusions

(1) Unlike the existing simple shear specimen, the proposed “double pull” specimen can avoid entire FRP-to-concrete interface debonding even under the load carrying capacity of FRP. The proposed “double pull” specimen, therefore, is more suitable for measuring δ-τ relationship.

(2) Under the given experimental conditions of this study, the ultimate slip δ_u is 0.31-0.52 mm.

(3) Extensometers should be incorporated in the future research in order to obtain more accurate δ-τ relationship.

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Foundation item: Project(2006BAJ03A07) supported by the National Key Technologies R & D Program of China; Project(5008283) supported by the Natural Science Foundation of Guangdong Province, China

Received date: 2009-04-21; Accepted date: 2009-07-26

Corresponding author: XIONG Guang-jing, PhD, Professor; Tel: +86-754-82902989; E-mail: gjxiong@stu.edu.cn

(Edited by YANG You-ping)